ULTIMATE EARTHQUAKE RESISTANCE OF STEEL OFFSHORE STRUCTURES

The survivability of steel offshore towers, in the event of severe subsea seismic ground excitations, depends on their cyclic inelastic behavior. In this dissertation experimental results are presented and interpreted on the cyclic inelastic behavior of two one-sixth scale frame models of a representative Southern California offshore platform designed according to American Petroleum Institute wave and earthquake criteria. Experimental results are also presented from tests of six brace specimens simulating diagonal braces in the frame models. The primary objective of the research effort is to improve the understanding of the behavior of braced structural systems subjected to damaging earthquake motions.; The 29.5 ft tall frame models consist of three braced panels which comprise a complete bent from the prototype platform. The displacement history imposed at the deck level with the base held stationary on both specimens is representative of severe seismic excitations. Two types of detailing representative of current practice, are investigated. In one specimen, the nominal tube diameter-to-wall thickness (D/t) ratio is 33, and in the other, it is 48. Observations are presented on: when the primary buckling of the brass occurred; which members were affected; the extent of damage as it progressed through the structure; and the onset of local buckling in members. Data from frame lateral force vs. lateral displacement hysteretic loops are examined as is the deterioration of the frame strength and stiffness. Energy dissipation and the effect of D/t ratio on the deterioration of strength are accorded special attention. The axial force-displacement response of frame bracing members is compared to those of the brace specimens with idealized pinned and fixed end conditions. These experiments provide data on the overall inelastic behavior of tubular braced frames which can be used to verify and extend mathematical models for predicting the severe seismic response of offshore structures.; The main element required to maintain the overall structural integrity of a braced structure subjected to lateral forces is the brace. To complement the frame model tests, six individual brace specimens are subjected to cycles of compressive inelastic buckling followed by tensile stretching. Two idealized bounds on the possible end conditions within a structure are considered: both ends pinned and both ends fixed. Also, the behaviors of heat treated braces are compared to the behavior of braces made from tubing as received from the manufacturer. The inelastic responses of the brace specimens are presented and interpreted. These include axial force-displacement hysteretic loops and axial force-midspan deflection hysteretic loops. Special attention is directed to the deterioration of buckling load with inelastic cycling. The reduction in buckling load can be attributed, in part, to changes in the mechanical material properties. A method of predicting these reductions is suggested.; Analytical methods of predicting the nonlinear frame and brace behavior are examined. The principal parameters and deformations required to model the cyclic inelastic behavior of a brace are identified. A physical theory model is formulated employing material plasticity theory and the effective length concept.; The frame and brace experimental results are examined with respect to the American Petroleum Institute's recommended design criteria. Data and general observations regarding the inelastic behavior of tubular frames and braces are offered as a means to extend the state of knowledge of the seismic response of offshore structures.